This invention pertains to apparatus and methods for illuminating a sample, such as a semiconductor wafer or photomask in an inspection system. It also pertains to apparatus and methods for reducing speckle effects during such illumination and inspection.
A diverse number and type of inspection systems are available for inspecting samples for defects. Lasers are often used as light sources in many inspection systems to detect defects on wafers or photomasks. Lasers provide one of the most efficient illumination methods due to their extremely high brightness.
One of the downsides, however, of using lasers is that the high spatial and temporal coherency of laser light can cause a ringing effect when imaging patterns on the surface of a sample, or speckle when the surface features are random. The ringing effect or speckle can severely degrade image quality and introduce excessive noise, therefore reduce sensitivity for detecting defects. Comprehensive discussions about interference effects such as ringing and speckle phenomena can be found in “Fourier Optics”, by J. W. Goodman, McGraw-Hill, and “Statistical Optics”, also by J. W. Goodman, Wiley-Interscience. This illumination spatial coherence effect is especially strong in a bright field imaging mode, where a significant portion of the reflected light needs to impinge on the detectors.
These deleterious image effects can be improved by reducing the spatial coherence of the laser light that is used to illuminate the object being imaged. One conventional technique of providing partially incoherent laser light involves the use of a rotating diffuser. A rotating diffuser typically consists of a rotating ground-glass screen that is introduced into the path of the laser beam before it reaches the object being imaged. The rotating diffuser introduces random phase variations into the incident laser beam, thereby reducing the spatial coherence of the beam. As the diffuser rotates, a detector can collect images of the object from independent views or perspectives. The detector, in turn, can integrate the independent inspection views to effectively synthesize an incoherent illumination of the object being imaged. One problem associated with using a rotating diffuser involves illumination efficiency. Diffusers are generally low efficiency because of excessive scattering of light.
In certain applications, it is also frequently required that the inspection system have configurable illumination and imaging configurations. The illumination and imaging configuration will be set to optimize the capture of different characteristics of defects or defect types. That is, different illumination and imaging configurations are more suitable for different types of defect inspections. Two broad categories of inspection configurations include bright field and dark field inspection. In general, the illumination and collection beam profiles are adjusted to achieve different inspection modes. In other words, different portions or angles of the incident or collection beam are blocked or transmitted. For example, in ring dark field mode, more than 75% of light can be lost due to the use of annular apertures to block the complementary portions of illumination pupil and imaging pupil. This can be a serious concern since the amount of incident light as part of the overall light budget for most laser inspection systems is limited. This is especially true for deep UV inspection systems where the light budget is extremely restricted.
In light of the foregoing, improved mechanisms for illuminating a sample, such as during an inspection, are needed.